Forty years ago today, brainwashed members of Charles Manson’s “Family” embarked on one of the most bizarre killing spree in recorded history.

From Aug. 9-10, 1969, seven people were brutally slaughtered in wealthy LA enclaves, and messages — one of which read “Helter Skelter” — were left smeared in blood for the cops.

Among the victims: the 81⁄2-months pregnant Sharon Tate, wife of film director Roman Polanski.

Today, Manson is 74 and serving a life sentence for conspiracy to commit the murders. Seven other members of the family are still incarcerated.

One follower, Lynette “Squeaky” Fromme, who was not linked to the murders, was later jailed for attempting to assassinate President Gerald Ford in 1975 — and is slated for release next Sunday.

Other family members are attempting to grow old in anonymity; many have changed their names, and several have become born-again Christians.

On the 40th anniversary of the Manson murders, The Post spoke to participants and observers to explain how these crimes came to pass, how they forever changed the culture, and what it was about Charles Manson that convinced ordinary suburban kids to kill for him.

Dennis Rice, 70, former Family member, born-again Christian minister:

I joined the Family shortly after the murders, after Manson was on the cover of Life magazine. I was always a revolutionary to begin with.

I thought this was a revolution sweeping America and that was going to take over the world: sex and drugs and rock ‘n’ roll. We had tried demonstrating against the injustices of the system, so to my way of thinking, the killings were just the next logical step.

They were an unfortunate tragedy that was forced upon the movement to make their point.

I was 31 years old when I joined. I had a very stable childhood, no abuse. It was my rebellion. That’s what boggled people’s minds about the Manson Family — they didn’t grow up in abusive homes.

I rolled up with my kids — my youngest was 2 or 3, the oldest was 10 — we stayed for dinner, and before you knew it, we spent a year and a half living with them. My kids loved it there.

In retrospect, I thought, “Oh my goodness, I wonder if they were abused.”

We all did what we wanted to do, and Charlie was just the vehicle. It looked like fun: acceptance, brotherhood, love, lack of responsibility, sex, drugs. I think Charlie is quite a unique person. I was excited to meet him.

I thought, “Here’s this guy who all these people adore.” I was impressed. His — I don’t want to say wisdom, it’s more his insights into life. He spoke in parables, like Jesus, to speak in basic truths.

He would qualify for what Reader’s Digest used to call, in their special serialized edition, “The Most Unforgettable Person I Have Ever Met.” He’s a genius.

I do not communicate with those who are in prison. It’s detrimental to them. The state would use that as, “Oh, they’re still in touch, still part of the same group.” I tried to get in touch with Lynette [“Squeaky” Fromme]. She wrote me back and said, “I don’t think it’s appropriate to communicate at this time.”

I hear she’s still very supportive of Charlie.

Gay Talese, who wrote a March 1970 story in Esquire about Manson’s headquarters, the Spahn Ranch outside LA:

I met some of the people who were murdered: [Coffee heiress] Abigail Folger, [Polish actor and writer] Wojciech Frykowski, and a hairdresser [Sharon Tate’s friend and former lover Jay Sebring]. I met them in my home in New York.

It was such an outlandish event. It seemed that, even by the standards of California and the movie world, it was so beyond the imagination of what was being filmed and what was being conceived as script material.

It parallels, of course, the Cutter family that was the basis of [Truman] Capote’s book “In Cold Blood.” The fact that the plot was even verified by factual accounts defied reality; it wouldn’t have worked as a work of fiction had it not been factual. Similarly, the Sharon Tate story: How could this happen?

Vincent Bugliosi, Manson prosecutor, co-author, “Helter Skelter”:

I was assigned the case in November; it was the biggest of my career. I was 35, married with two little kids. There were only two fingerprints left at the Tate house — it was viewed as a weak case. These were strange, brutal murders. They seemed to be random. No valuables were taken.

Their nightmarish nature — young women dressed in black, armed with sharp knives, entering the homes of complete strangers in the middle of the night and committing an orgy of murder — this is stuff you don’t see in horror movies.

People were dropped from guest lists, parties were canceled, because no one knew if the killers were among them. This was in Bel Air and Beverly Hills. The sale of guns and guard dogs rose dramatically.

That fright and shock increased when the killers were found, and they turned out to be hippie kids with average American backgrounds. Leslie Van Houton was a homecoming princess. The murders sounded the death knell for the hippie era.

The evidence was circumstantial, so with Manson, it became, “How do I connect him?”

He did not physically participate. He was not present at the time of the murders. I had to prove motive and domination. I proved through witnesses that only Manson had the motive, and it was his philosophy, “Helter Skelter.”

He wanted to start a race war. He introduced it to the Family, and talked about it all the time. He was a dictatorial figure, the king. [The Family] was slavishly obedient to him. If I could prove his co-defendants committed the murders, then I could prove that he masterminded them.

This is probably the most bizarre case in the recorded annals of crime. Sinatra, Liz Taylor, Richard Burton, Steve McQueen, Tom Jones — all were on Manson’s list.

I was assigned a bodyguard when it was learned that I was on his death list, but I don’t want to get into that. [It’s been said] that they may have been responsible for 35 murders. I feel, without knowing, that they are, and that those bodies are out there in the desert.

I don’t think Manson was mentally ill. I believe he’s crazy in a layman’s sense, not legally. He’s crazy like Hitler, who was Manson’s biggest hero.

Manson was able to assess his weaknesses and strengths. He used his minions to vent his spleen on society. He felt he was not dealt a fair hand by life. He’s short — 5-foot-2. He didn’t know his father, his mother drank, would disappear for weeks or months.

I don’t know the exact genesis of all this, but there’s no question he hated the establishment.

He is also very, very bright. He got other people to kill at his command. The fright that his followers generated was finite. But Manson got other people to kill for him, so the fright becomes exponential. And people like to be frightened. Don’t ask me why.

James Poniewozik, Time magazine TV critic:

The killings seemed to crystallize a moment when “the ’60s” — as we associate them culturally with freedom, love, counterculture, etc. — started to turn sour.

The fact that Manson had an association, however tenuous, with the music industry — through Beach Boy Dennis Wilson, for example, and appropriating the term Helter Skelter — only underscored that. It kind of contributed to the sense of that mid-’60s/Summer of Love California dream turned overripe and rotten.

Sasha Frere-Jones, New Yorker music critic:

Sonic Youth’s song “Death Valley 1969” was the beginning of a hip veneer for Manson. It came out in 1985. They did a video where they re-enacted the Sharon Tate murders in a very comic way — Thurston Moore was in the bathtub and Kim holding a shotgun.

It was the beginning of a certain moment where kind of questionable stuff started to become hip, a moment in indie rock when everyone was kind of trying to be more scary than the other guy. Everyone was writing about murder and rape and child molestation.

Everyone trying to do the freakiest thing they could do. Manson became a bona fide in a weird way. I thought that whole thing was kinda sketchy. Then it became the highly successful ’90s mainstream thing, and the biggest part of it was Marilyn Manson and Nine Inch Nails.

It was a moment where in a very sort of teenage way it was cool and creepy to talk about him, but then it was over because a) it’s wrong to kill people and b) it seemed like a kind of cheesy thing to do.

Ed Sanders, author, “The Family,” a detailed account of Manson and the murders:

Manson is basically a hillbilly psychopath from West Virginia who won the Most Hideous Childhood in America contest of 1934 when his mother tried to sell him for beer.

Through a process of natural selection, he found psychopaths who were willing to rob banks and kill people. He was like somebody running an off-Broadway play called “Will You Kill For Me?” And he started casting all through the late ’60s.

He learned how to run hookers on Wilshire Boulevard in the ’50s. Then he burrowed his way into the Beach Boys, this great thing. He hated Johnny Mathis because of his color, but tried to sing like Johnny Mathis. He was a typical guy of the ’60s — he thought because you had a record contract you were already rich.

If there had been cellphones, there would never have been a Manson family. Some mother would’ve called and said, “Where are you? I’m calling the FBI.” Those cellphones would’ve interrupted a lot of orgies.